The input data stream of an optical transmission system may be viewed as a series of light pulses representing digital bits. The bit rate of current optical transmission systems generally range from 10 GHz to 40 GHz resulting in light pulses (or bit periods) that are, respectively, 100 to 25 picoseconds wide. Receivers in an optical transmission system convert each bit period in the data stream into digital ones or zeros by determining, for each bit period, whether a light pulse has been received (digital one) or not (digital zero). Polarization mode dispersion (PMD) is a phenomenon that may distort the light pulses of the data stream and thus impair the ability of a receiver to determine whether a bit period should be converted into a one or zero. As a result, PMD limits the transmission accuracy and capacity of optical transmission systems.
Polarization mode dispersion arises from birefringence of the transmission medium of an optical transmission system. Birefringence is present in transmission medium comprised of even so called “single-mode” optical fiber because of fiber imperfections and asymmetric stresses that result in a noncircular fiber core. An ideal single-mode optical fiber has a circular core, i.e., the core is isotropic and without eccentricity. Such an ideal fiber is isotropic, that is, the refractive index of the fiber is independent of the orientation of the electric field or, equivalently, the polarization of the light. Anisotropy (e.g., eccentricity) in an optical fiber core leads to birefringence and, therefore, different polarizations of light propagate through the optical fiber at different velocities.
Light propagation in optical fiber may be viewed as governed by two fundamental or principal modes. These principal modes are known as “principal states of polarization” (“PSPs”). If a PSP is introduced into a fiber link, the polarization at the output of the link will be substantially constant to first order in frequency. In an ideal single-mode fiber the PSPs are degenerate, i.e., indistinguishable. Anisotropy of the fiber core lifts this degeneracy. As a result, the PSPs travel at different group velocities and separate into two temporally displaced pulses. The separation of the PSPs due to different group velocities is known as polarization mode dispersion (PMD), and the temporal spread between the two PSPs is known as the “differential group delay” (“DGD”). This temporal spreading can cause the light pulse of one bit period in the data stream to overlap with another bit period. This overlap impairs the ability of a receiver to determine whether a bit period should be converted into a one or zero. Consequently, PMD is a problem for optical transmission systems that results in data ambiguity, data loss, data corruption, and limited transmission capacity.
While various approaches to the PMD problem have been proposed, each presents limitations. For example, polarization-maintaining fiber is designed to maintain the input polarization through inherent optical properties, such as stress-induced anisotropy introduced by internal stress members within the fiber that cause birefringence and prevent cross-coupling of optical power between the PSPs. Unfortunately, this specialty fiber is not only expensive, but, short of wholesale replacement, cannot address PMD in existing “legacy” fiber networks.
Present electronic approaches, such as electrical distortion equalizers, also exhibit disadvantages. These approaches, which typically use a notch in the RF frequency response (i.e., response minima) at the receiver as an indicator of DGD, require modifications to conventional receiver electronics and tend to require high-speed digital or RF electronics.
Optical measurement approaches typically require either perturbing the laser source by polarization scrambling or by the introduction of frequency sidebands, or provide only indirect or qualitative measures of the polarization properties of the PMD. In an optical transmission system, perturbing the laser source for optical measurements is generally not practical and interrupts data transmission. Approaches that utilize only indirect or qualitative measures of PMD polarization properties, such as DGD and degree of polarization (“DoP”) measurements, require use of an iterative procedure that compensates for PMD only after multiple operations. However, such multiple operations are time consuming; and thus, such iterative compensation approaches have drawbacks for application to high-speed transmission systems.
A need therefore exists for an approach providing a reliable measurement of PMD that avoids interrupting data transmission, and that allows for faster compensation of PMD effects.